Control systems for stabilizing a turning condition of a vehicle are known in the art. The conventional control systems are configured to adjust a turning condition exemplified by a yaw rate to a target turning condition determined based on a vehicle speed and a steering angle by controlling a driving force (or a braking force) of the vehicle. Given that a driving force rotating rear wheels is reduced during turning by reducing an engine power or increasing a braking force applied to the rear wheels, a load applied to front wheels being steered is increased and hence a lateral force acting on the front wheels is increased thereby increasing a yaw of the vehicle. To the contrary, given that the driving force rotating the rear wheels is increased, the yaw is reduced. Specifically, the yaw of the rear wheel layout vehicle is increased by rotating an outer rear wheel faster than an inner rear wheel during turning. To the contrary, the yaw of the rear wheel layout vehicle is decreased by reducing a speed difference between the outer rear wheel and the inner rear wheel during turning.
Japanese Patent Laid-Open No. 2005-88875 describes a motion control device for controlling a driving force and a braking force applied to wheels to stabilize a turning behavior of a vehicle even if a brake is operated during turning. According to the teachings of Japanese Patent Laid-Open No. 2005-88875, when a braking operation is detected during turning, a control ratio of the braking force applied to drive wheels and the driving force rotating the driving wheels generated by an engine is equalized, and then, the control ratio of the driving force generated by the engine is increased.
Japanese Patent Laid-Open Nos. 2000-203300 and 2000-203299 describe vehicle controllers for stabilizing a turning performance of a vehicle until reaching the performance limit governed by a friction coefficient etc. According to the teachings of those prior art documents, a driving force of the engine is reduced depending upon an increment of a speed difference between the right and the left wheels, or upon a mechanical limit exceedance of a target power ratio to the right and left wheels.
As described, a control device taught by Japanese Patent Laid-Open No. 2005-88875 is configured to improve turning performance of a vehicle by equalizing the control ratio between the braking force applied to the wheels and the driving force rotating the wheels upon execution of the braking operation during turning, and then increasing the ratio of controlling the driving force. Specifically, if the vehicle does not turn enough, the driving force applied to the wheels is increased and the braking force applied to the inner wheel is reduced. By contrast, if the vehicle turns more sharply than intended, the driving force applied to the wheels is reduced and the braking force applied to the inner wheel is increased. Then, the ratio of controlling the driving force is increased after controlling the driving force and the braking force. Thus, according to a control device taught by Japanese Patent Laid-Open No. 2005-88875, the control ratio between the braking force applied to the wheels and the driving force rotating the wheels is equalized upon execution of the braking operation during turning to improve the turning performance, and then the driving force is increased. If the driving force is increased while applying the braking force to the wheels under the condition that the vehicle does not turn enough, the fuel may be consumed wastefully.